|BMW R75, R51 & RS253
Following the lead of the English and Scandinavians with their 500 cc Formula in 1946, the Germans adopted small car regulations and recommenced racing in 1948, a remarkably short period after the end of the war. The German Kleinstrennwagen regulations specified 750cc for the pragmatic reason that there were rather more engines to this capacity. Most notable was the BMW R75 which, in military sidecar specification was readily available, and adopted by the majority of constructors. The other common engines, at least in the kleinstrennwagen period, was the two stroke DKW and Zuendapp twin.
The R75 was developed as a sidecar machine for the Wehrmacht and went into production in 1941, lasting through to 1944. It featured a horizontally opposed flat twin arrangement, initially with side vale heads but these were quickly replaced with a new overhead valve head operated by pushrods. Bore and stroke were 78mm by 78mm with 5.8:1 compression ratio In standard form the R75 gave 29 hp, a significant disadvantage to the 48 hp of the competition bred JAP. Tuning, for example with camshafts made by Rudolf Schleicher, increased the power to almost the same level as a JAP.
For 1950, when the 500 cc Formula 3 came into force, Germany ran both classes side by side for a period while competitors modified their engines, by means of a liner, to reduce the capacity to 500cc. This made the engine rather under square which wouldn't have helped it to rev, not ideal in a competition engine.
R5 or R51
With the move to Formula 3, some competitors used the 1936 R5 (later R51), an engine designed as a 500 cc with square dimensions of 68mm x 68mm and therefore needing no modifications, although it only gave 24 hp in standard trim.
BMW offered this engine in Super Sport specification with higher compression, bigger carburettors and modified alternator drive, though still fundamentally a production engine, and in more radical and rare, Renn Sport specification with aluminium barrels and modified valve actuation though still essentially pushrod. In this RS specification, the R51 gave about 36 hp, closer but still short of the JAP and Norton but the low centre of gravity helped handling when mounted in a car.
It seems that only a few favoured drivers (including "Fips" Meub, Friedrich Staschel and Siegfried Seifert) obtained RS 253 racing engines in 1958 (the engine was introduced in 1954 for solo and sidecar racing), and Seifert had the rarest of them all a "Prototyp" from 1952, in long stroke form of 66mm x 72mm This fully race oriented engine was then changed to short stroke over square specification with bore and stroke of 70mm x 64.5mm, high compression, valves actuated by bevel shafts and twin cams, twin plugs and two Dell'Orto carburettors. Power was allegedly up at the 50 hp mark at a heady 9,000 revs. Sadly, Formula 3 racing ceased in West Germany just as the engine arrived so it only really featured in East Germany and never got the opportunity to show against the Cooper-Nortons at International level.
Known BMW powered cars include:
The BMW engine would have another story in Formula III. Early in World War II, the Russians had built the IMZ plant to produce BMW-design motorcycles under licence (the R71 model, predecessor of the R75). This, of course, used a version of the boxer motor. Production continued post-War as a civilian bike (albeit now rather primitive), before plans were made to produce a modernised machine. The Serpukhov All-Union Research Facility of the Motorcycle Industry was tasked with developing the engine, and the design they came up with in 1954 bore an uncanny resemblance to the BMW R51/3. The IMZ-Ural M52 went into production in 1957. Just as in Germany, it was the go-to engine for almost all of the Soviet Formula III builders, including Zvezda and Estonia.
Our thanks to Lothar Mildebrath